[PATCH] Lower certain build_vectors to insertps instructions

[Filipe Cabecinhas]

Hi nadav, delena, craig.topper,

Vectors built with zeros and elements in the same order as another
(source) vector are optimized to be built using a single insertps
instruction.

Further optimizations are possible, described in TODO comments.
I will be implementing at least some of them in the near future.

Added some tests for different cases where this optimization triggers.

http://reviews.llvm.org/D3521

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/sse41.ll
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